1. Field of the Invention
The present invention relates to a magnetic recording material and, more particularly, to a high density magnetic recording material having excellent adhesion between the support and the magnetic recording layer, and excellent running property. The present invention is useful in the fields of video tapes, flopp discs, and others.
2. Description of the Prior Art
High density magnetic recording materials, in particular audio cassette tapes or video cassette tapes, are restricted as to the length of tape by the size of the cassette half. In order to increase recording time the tape speed is slowed down and shorter wavelengths are used to record on the tape. There is a limit on the wavelengths which may be recorded on tapes, and the use of shorter wavelengths for recording on conventional magnetic recording materials has inevitably led to a reduction in sound or recording image quality.
Conventional magnetic recording materials comprise a magnetic recording layer and a non-magnetizable flexible support. The non-magnetizable flexible support usually has a thickness two times or more that of the magnetic recording layer, though it does not directly participate in magnetic recording. By reducing the thickness of the non-magnetizable flexible support it might be possible to record for longer periods of time using the same tape speed and the same cassette half or cartridge and record with the same sound or image quality. Thus, a reduction in thickness enables one to raise recording density per volume of magnetic recording tape. This approach is extremely practical due to the interchangeability between the thus recorded cassette tapes and conventional ones. However, when reduced in thickness, conventional non-magnetizable flexible supports suffer a reduction in tape stiffness and a deterioration in running property due to low Yound's modulus. Thus, it has been difficult to make the thickness of the tape thinner than the present level, e.g., 6 .mu.m in Philip's type compact cassette tapes.
In recent years aromatic polyamide supports having higher Young's modulus than polyethylene terephthalate (hereinafter PET) which is a prominent non-magnetizable support have been proposed and described in Japanese Patent Application (OPI) Nos. 98897/74 (The term "OPI" as used herein refers to a "published unexamined Japaneses patent application"), 81880/76, 68660/76 and 129201/76 and U.S. Pat. No. 4,112,187, etc. These aromatic polyamide supports have a Young's modulus of not less than 1,500 kg/mm.sup.2, which is two times or more that of PET (500 to 800 kg/mm.sup.2). Cassette tapes prepared using aromatic polyamide supports have poor adhesion between the support and the magnetic recording layer. Such adhesion is particularly poor in cassette tapes in which an isocyanate hardener is used as a part of the recording layer binder. As a result there is such a drop-out phenomenon that aromatic polyamide supports have not been put into practice. This may be due to the stronger hydrogen bond strength and the greater solubility parameter of aromatic polyamides than vinyl chloride resin, vinyl acetate resin, polyurethane resin, etc., generally used as a binder.